As you explore the world, you'll perhaps stumble across the occasional secret memo. Each one you might discover is located well off the beaten path, so you're not likely to find any of them by accident unless you are exploring quite thoroughly. There are actually a total of ten memos available. You can access them all and use their contents to assemble a secret code that you can enter while using the Wii U browser, on Miiverse. Doing so will allow you to see special video logs from the game's characters.
The codes that you can enter to view the videos are as follows:
0934550498 - In the Garden
8554536398 - In the Wilds
1396339547 - In the Tundra
4236786831 - At the River
1734517538 - At the Oak
The codes above may vary depending on the Wii U system that is used to access them. There may be a limit to the number of times that each code may be used to access the corresponding video. If you would like to find the notes yourself, and assemble the text to create your own code that you can enter on Miiverse, you should find the following information and videos about individual locations useful (note that the numbering system used below is for your convenience and is not referenced in the game itself).
Secret Memo 1 is hidden atop a grass-covered mesa in the Tropical Wilds area on the western end of the map. A large brown square on the ground near a ledge serves as an elevator. To activate it, place some crew members and a few Pikmin on top, then send another crewer around the ridge until you see a bounce mushroom hidden by plants. Throw some Pikmin onto the mushroom and they'll bounce on a trigger which elevates the stone onto the far side where your crew members are waiting. A single prominent tree branch blocked by a small mushroom serves as a bridge from the mesa to a hidden area where the memo awaits.
Secret Memo 2 is hidden in the Distant Tundra near the Cupid's Grenade atop a snowman's head.
Secret Memo 3 is hidden in a flowerpot on the island near the boss lair in the Garden of Hope.
Secret Memo 4 is hidden in the Twilight River area beneath a pile of bridge tiles on a tiny jutting plot of land in the southern part of the map.
Secret Memo 5 is hidden above the scales in the Garden of Hope.
Secret Memo 6 is hidden in the roots of the great tree near the Scornet boss fight in the Twilight River stage. The Memo can not be seen from the main map. To reach it, throw a crewer over the edge of the east edge of the tree root path just past the bridge and ravine leading up to the boss. The Memo is hidden just past a large tree root.
Secret Memo 7 is hidden in the Formidable Oak. Just on the far side of the concrete barrier near the cavern entrance you'll see a cluster of small mushrooms. Break them down to reveal the Memo.
Secret Memo 8 is hidden in the cavern in the northwest section of the Distant Tundra across the Bamboo Gate near the open circuit.
Secret Memo 9 is hidden in the Tropical Wilds at the far southwest corner of the map, tucked away in a small underwater grotto.
Secret Memo 10 is hidden in the Formidable Oak. The note is buried in a dirt pile just to the left of the final ascent to the boss battle in the dark cave area.